This invention relates to semiconductor junction lasers.
In the development of integrated optical devices the need has long been recognized for the efficient division of optical power between two or more waveguides. The signal in one waveguide might be a carrier modulated with telecommunication information, for example, whereas the signal in the other waveguide might be employed in a feedback scheme to stabilize the output of the laser used to generate the carrier.
One method for achieving power division in different waveguides utilizes phase coupling between a pair of synchronous waveguides. For example, many different schemes have been devised for electro-optic phase-coupled switching in LiNbO.sub.3 and GaAs devices. Another approach, with less-stringent fabrication tolerances, involves the use of branching waveguides which either divide the optical power between two (or more) channels, or separate different transverse modes in the incident guide. A paper by H. Yajima in Applied Physics Letters, Vol. 22, p. 647 (1973) is illustrative. Yajima fabricated glass-SiO.sub.2 waveguides and established the principal that an incident mode would choose the branch having a propagation constant closest to that of the propagation constant of the incident mode. A more complete theory of branching waveguides was developed later, by W. K. Burns and A. F. Milton, IEEE Journal of Quantum Electronics, Vol. QE-11, p. 32 (1975), who considered a planar, 5-layer theoretical structure including two waveguiding layers (2, 4) and three layers (1, 3, 5) of lower refractive index that separated and clad the two waveguiding layers. They analyzed the situation of a taper in the cladding layer separating the waveguides by a step approximation method and showed that the behavior of such a branching waveguide could be described by a transition parameter A: EQU a=.vertline..beta..sub.4 -.beta..sub.2 .vertline./.theta.(.beta..sub.0.sup.2 -.sub.3.sup.2 k.sup.2).sup.1/2, (1)
where .beta..sub.i (or n.sub.i) is the propagation constant (or refractive index) of layer i (i=2, 3, or 4), in the region where the waveguide layers are separated by the taper, .beta..sub.0 =(.beta..sub.2 +.beta..sub.4)/2, k=2.pi./.lambda., .lambda. is the wavelength of the incident mode, and .theta. is the angle of the branching taper. If A.ltorsim.0.4 (large .theta., symmetric waveguides), mode conversion occurs and the device functions as a TPD. If A.gtorsim.0.4, (small .theta., asymmetric guides), modal evolution occurs through the branching region, and the device is a TMS.